[PATCH] remove /proc/sys/vm/dirty_sync_thresh

This was designed to be a really sterm throttling threshold: if dirty
memory reaches this level then perform writeback and actually wait on
it.

It doesn't work.  Because memory dirtiers are required to perform
writeback if the amount of dirty AND writeback memory exceeds
dirty_async_ratio.

So kill it, and rely just on the request queues being appropriately
scaled to the machine size (they are).

This is basically what 2.4 does.

Documentation/filesystems/proc.txt

@@ -963,13 +963,6 @@ Contains, as a percentage of total system memory, the number of pages at which
 a process which is generating disk writes will itself start writing out dirty
 data.
 
-dirty_sync_ratio
-----------------
-
-Contains, as a percentage of total system memory, the number of pages at which
-a process which is generating disk writes will itself start writing out dirty
-data and waiting upon completion of that writeout.
-
 dirty_writeback_centisecs
 -------------------------
 

Documentation/sysctl/vm.txt

@@ -21,13 +21,12 @@ Currently, these files are in /proc/sys/vm:
 - dirty_async_ratio
 - dirty_background_ratio
 - dirty_expire_centisecs
-- dirty_sync_ratio
 - dirty_writeback_centisecs
 
 ==============================================================
 
 dirty_async_ratio, dirty_background_ratio, dirty_expire_centisecs,
-dirty_sync_ratio dirty_writeback_centisecs:
+dirty_writeback_centisecs:
 
 See Documentation/filesystems/proc.txt
 

fs/fs-writeback.c

@@ -262,9 +262,6 @@ sync_sb_inodes(struct super_block *sb, struct writeback_control *wbc)
 			break;
 
 		really_sync = (wbc->sync_mode == WB_SYNC_ALL);
-		if ((wbc->sync_mode == WB_SYNC_LAST) && (head->prev == head))
-			really_sync = 1;
-
 		BUG_ON(inode->i_state & I_FREEING);
 		__iget(inode);
 		list_move(&inode->i_list, &sb->s_dirty);

include/linux/sysctl.h

@@ -147,12 +147,11 @@ enum
 	VM_PAGE_CLUSTER=10,	/* int: set number of pages to swap together */
 	VM_DIRTY_BACKGROUND=11,	/* dirty_background_ratio */
 	VM_DIRTY_ASYNC=12,	/* dirty_async_ratio */
-	VM_DIRTY_SYNC=13,	/* dirty_sync_ratio */
-	VM_DIRTY_WB_CS=14,	/* dirty_writeback_centisecs */
-	VM_DIRTY_EXPIRE_CS=15,	/* dirty_expire_centisecs */
-	VM_NR_PDFLUSH_THREADS=16, /* nr_pdflush_threads */
-	VM_OVERCOMMIT_RATIO=17, /* percent of RAM to allow overcommit in */
-	VM_PAGEBUF=18		/* struct: Control pagebuf parameters */
+	VM_DIRTY_WB_CS=13,	/* dirty_writeback_centisecs */
+	VM_DIRTY_EXPIRE_CS=14,	/* dirty_expire_centisecs */
+	VM_NR_PDFLUSH_THREADS=15, /* nr_pdflush_threads */
+	VM_OVERCOMMIT_RATIO=16, /* percent of RAM to allow overcommit in */
+	VM_PAGEBUF=17,		/* struct: Control pagebuf parameters */
 };
 
 

include/linux/writeback.h

@@ -27,10 +27,9 @@ static inline int current_is_pdflush(void)
  * fs/fs-writeback.c
  */
 enum writeback_sync_modes {
-	WB_SYNC_NONE =  0,	/* Don't wait on anything */
-	WB_SYNC_LAST =  1,	/* Wait on the last-written mapping */
-	WB_SYNC_ALL =   2,	/* Wait on every mapping */
-	WB_SYNC_HOLD =  3,	/* Hold the inode on sb_dirty for sys_sync() */
+	WB_SYNC_NONE,	/* Don't wait on anything */
+	WB_SYNC_ALL,	/* Wait on every mapping */
+	WB_SYNC_HOLD,	/* Hold the inode on sb_dirty for sys_sync() */
 };
 
 /*
@@ -65,7 +64,6 @@ static inline void wait_on_inode(struct inode *inode)
 /* These 5 are exported to sysctl. */
 extern int dirty_background_ratio;
 extern int dirty_async_ratio;
-extern int dirty_sync_ratio;
 extern int dirty_writeback_centisecs;
 extern int dirty_expire_centisecs;
 

kernel/sysctl.c

@@ -292,9 +292,6 @@ static ctl_table vm_table[] = {
 	{VM_DIRTY_ASYNC, "dirty_async_ratio", &dirty_async_ratio,
 	 sizeof(dirty_async_ratio), 0644, NULL, &proc_dointvec_minmax,
 	 &sysctl_intvec, NULL, &zero, &one_hundred },
-	{VM_DIRTY_SYNC, "dirty_sync_ratio", &dirty_sync_ratio,
-	 sizeof(dirty_sync_ratio), 0644, NULL, &proc_dointvec_minmax,
-	 &sysctl_intvec, NULL, &zero, &one_hundred },
 	{VM_DIRTY_WB_CS, "dirty_writeback_centisecs",
 	 &dirty_writeback_centisecs, sizeof(dirty_writeback_centisecs), 0644,
 	 NULL, &proc_dointvec_minmax, &sysctl_intvec, NULL,

mm/page-writeback.c

@@ -72,11 +72,6 @@ int dirty_background_ratio = 10;
  */
 int dirty_async_ratio = 40;
 
-/*
- * The generator of dirty data performs sync writeout at this level
- */
-int dirty_sync_ratio = 50;
-
 /*
  * The interval between `kupdate'-style writebacks, in centiseconds
  * (hundredths of a second)
@@ -105,15 +100,11 @@ static void background_writeout(unsigned long _min_pages);
  * - Does nothing at all.
  *
  * balance_dirty_pages() can sleep.
- *
- * FIXME: WB_SYNC_LAST doesn't actually work.  It waits on the last dirty
- * inode on the superblock list.  It should wait when nr_to_write is
- * exhausted.  Doesn't seem to matter.
  */
 void balance_dirty_pages(struct address_space *mapping)
 {
 	struct page_state ps;
-	long background_thresh, async_thresh, sync_thresh;
+	long background_thresh, async_thresh;
 	unsigned long dirty_and_writeback;
 	struct backing_dev_info *bdi;
 
@@ -122,20 +113,9 @@ void balance_dirty_pages(struct address_space *mapping)
 
 	background_thresh = (dirty_background_ratio * total_pages) / 100;
 	async_thresh = (dirty_async_ratio * total_pages) / 100;
-	sync_thresh = (dirty_sync_ratio * total_pages) / 100;
 	bdi = mapping->backing_dev_info;
 
-	if (dirty_and_writeback > sync_thresh) {
-		struct writeback_control wbc = {
-			.bdi		= bdi,
-			.sync_mode	= WB_SYNC_LAST,
-			.older_than_this = NULL,
-			.nr_to_write	= sync_writeback_pages(),
-		};
-
-		writeback_inodes(&wbc);
-		get_page_state(&ps);
-	} else if (dirty_and_writeback > async_thresh) {
+	if (dirty_and_writeback > async_thresh) {
 		struct writeback_control wbc = {
 			.bdi		= bdi,
 			.sync_mode	= WB_SYNC_NONE,
@@ -331,8 +311,6 @@ static int __init page_writeback_init(void)
 		dirty_background_ratio /= 100;
 		dirty_async_ratio *= correction;
 		dirty_async_ratio /= 100;
-		dirty_sync_ratio *= correction;
-		dirty_sync_ratio /= 100;
 	}
 
 	init_timer(&wb_timer);